What do most of us know about anodes. Well I know that our boats need them, and that Anodes are sacrificial, they protect your boat hull from galvanic corrosion where salt water attacks metal and causes it to deteriorate, and need to be replaced periodically; but as to anything more detailed I’ll have to be honest as say ‘I’m not entirely sure’. I also realise having chatted to friends that they are in a similar situation, so here goes, I’m going to chat about anodes in layman’s terms and hopefully demystify anodes.
I have already used the term ‘sacrificial’ this means the anodes waste away rather than other metals connected to the water on our boat, such as the steel of the prop shaft or the copper propeller.
Active metals are chosen for anodes, they corrode more easily because they are ‘softer’ (lower down the periodic table), than the compounds that make steel or copper. Anodes can be made from Zinc, aluminum or magnesium. The most commonly used for pleasure craft are Zinc, because they have the best electrical potential.
Here we have an example of how the anode has been sacrificed instead of the metal rudder.
The task anodes preform
Although anodes come in all shapes and sizes basically they all preform the same task, but they will not work unless the anode is connected to the area you wish to protect using a copper bonding wire. You need to wire your anode to the metal you wish to protect. This is the start of our ‘electrical circuit’.
If you think of your anode as the negative terminal on your battery, the battery needs to be connected electrically to the metal that needs your protection ( which is the positive terminal of this ‘battery’), for example the propeller or engine. It is no good just attaching an anode to the bottom of your boat, if the anodes are not connected properly, you might as well not have them.
Sean my husband is now going off to our boat to follow the wires from our anode and see what we are and aren’t protecting! We purchased the boat three years ago, and although we have replaced our anodes, we actually have no idea if and what they might be protecting.
This is how the corrosion occurs, the electrolyte which is for example sea water is more active on the anode metal, quite literally eating it away, whilst the positive connection the cathode is being protected. In order to provide adequate protection there should be a voltage differential between the metals of the anode, which should be the least active and the cathode which should have a high voltage. Zinc is thus used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller.
(http://www.mgduff.co.uk/leisure-craft/hull-anode-selection have a clear diagram that you can see.)
It should also be noted that the current flow protection is dependent on the area size of your anode, and the longevity on the anode mass. So in this instance…size does matter!
Anode erosion samples
This bonding wire may also be connected somewhere to the rigging. This is not for electrolysis protection but for some protection from lightning strikes to conduct it into the water through the items connected together.
Don’t mix your anode metals, because one will protect the other, rather than both protecting the cathodes. Its best to avoid painting the anodes…they wont work if covered.
Owners often mistakenly believe that coating of submerged steel eliminates the need for anodes. You still need to protect the areas around the prop as they can suffer from the presence of microscopic pinholes, which lead to the development of concentrated areas of corrosion. http://www.mcpsltd.com/. But antifouling steel isn’t the answer as the pinhole erosions will still occur beneath the paint, and you will not be able to see these areas. Sometimes ignorance is bliss, but not so in this instance.
Even using the anodes can mean that pitting like this can still occur. If you do decide to use paint in addition to anodes; do not apply it directly to the steel and use an anti corrosive primer first. Follow the manufacturers instructions to the latter.
Here are some further examples of shapes sizes and quantities of anodes used for protection
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Anodes can be purchased online from Mylor Chandlery , postage is £5.95 within the UK. Whilst you are considering purchasing new anodes don’t forget the backing sheet, and to check that the bolts are still in good order, before you place your order!
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